Cathelicidin-related antimicrobial peptides are a family of polypeptides produced by macrophages and polymorphonuclear leukocytes (PMNs), and epithelial cells such as keratinocytes. Cathelicidins serve a critical role in mammalian innate immune defense against invasive bacterial infection. The peptides of the cathelicidin family are classified as antimicrobial peptides (AMPs).
Cathelicidin peptides have been isolated from many different species of mammals, including but not limited to humans, monkeys, mice, rats, rabbits, guinea pigs, pandas, pigs, cattle, frogs, sheep, goats, chickens, and horses. Some species produce more than one cathelicidin. All cathelicidins are enriched for positively-charged amino acids, but the sequences for the cathelicidins produced by different animals have highly varied sequences.
Currently identified cathelicidins include but not limited to the following:                Human: hCAP-18/LL-37 (SEQ ID NO: 1)        Rhesus Monkey: RL-37 (SEQ ID NO: 2)        Mice: CRAMP (SEQ ID NO: 4), (Cathelicidin-related Antimicrobial Peptide)        Rats: rCRAMP        Rabbits: CAP-18 (SEQ ID NO: 3)        Guinea Pig: CAP-11 (SEQ ID NO: 5)        Pigs: PR-39, Prophenin, PMAP-23,36,37        Cattle: BMAP-27,28,34 (Bovine Myeloid Antimicrobial Peptides); Bac5, Bac7        Horses: eCATH-1 (SEQ ID NO: 13), eCATH-2 (SEQ ID NO: 10), eCATH-3 (SEQ ID NO: 9)        Frogs: cathelicidin-AL (found in Amolops loloensis)        Sheep: SMAP-29 (SEQ ID NO: 15)        Chickens: Four cathelicidins, fowlicidins 1,2,3 and cathelicidin Beta-1        
Cathelicidins range in size from 12 to 80 amino acid residues and have a wide range of structures. Most cathelicidins are linear peptides with 23-37 amino acid residues, and fold into amphipathic α-helices. In addition, some cathelicidins contain beta-strand structures or can be stabilized by one or two disulfide bonds. Larger cathelicidin peptides (39-80 amino acid residues) are also possible. These larger cathelicidin peptides display repetitive proline motifs forming extended polyproline-type structures.